March 21, 1994
12:00 AM (EST)

News Release Number: STScI-1994-15

Galaxy Drift Challenges Ideas About Universe's Evolution

The full news release story:

Two astronomers have discovered that our own Milky Way galaxy and most of its
neighboring galaxies, contained within a huge volume of the universe, one billion light-years
in diameter, are drifting with respect to the more distant universe. This startling result may
imply that the universe is "lumpier" on a much larger scale than can be readily explained by
any current theory. "The new observations thus strongly challenge our understanding of how
the universe evolved," says Dr. Tod Lauer of the National Optical Astronomy Observatories
(NOAO).

This surprising conclusion comes from the deepest survey of galaxy distances to date,
conducted by Dr. Tod R. Lauer in Tucson, Arizona, and Dr. Marc Postman of the Space
Telescope Science Institute (STScI) in Baltimore, Maryland.
The two astronomers used NOAO telescopes at Kitt Peak National Observatory, near Tucson,
Arizona, and at Cerro Tololo Inter-American Observatory, near La Serena, Chile to study
galaxy motions over the entire sky out to distances of over 500 million light years. They
explored a volume of space about thirty times larger than had been surveyed previously. The
results of this survey will be published in the April 20 issue of The Astrophysicd Journal.
The expansion of the universe causes all the galaxies in the volume surveyed to be moving
away from us. Galaxies at the edge of the volume are receding from us at 5% of the speed
of light. The large flow that Postman and Lauer discovered comes from looking at the galaxy
motions "left over" once the expansion of the universe had been accounted for. The flow
means that the nearby universe, as well as expanding, appears to be drifting with respect to
the more distant universe.

Astronomers generally assume that the diffuse glow of microwave radiation left over from the
Big Bang provides the backdrop or rest frame of the universe. In the mid 70's astronomers
found that temperature of this radiation is slightly hotter towards the direction of the
constellation of Leo.

This effect has been interpreted to mean that the Milky Way is drifting with respect to the
rest of the universe at about 380 miles per second in this direction. It has also been assumed
that most of this motion is due to the gravitational attraction of more distant galaxies;
however, these galaxies have never been positively identified.

In the mid-80's a group of seven astronomers surveyed the motions of galaxies out to about
one-third of the distance studied by Lauer and Postman, finding the galaxies to be flowing as
a group with respect to the more distant universe. This team postulated that this flow was
due to the gravitational pull of a large concentration of galaxies dubbed "The Great
Attractor." The Great Attractor is located deep inside the volume surveyed by Postman and
Lauer, however, and would not be massive enough to cause their much larger sample of
galaxies to drift.

In fact, the new result implies that the Milky Way and its neighbors are affected by much
larger concentrations of galaxies at much larger distances than can be easily explained by
popular theories of how the universe is organized.

Lauer and Postman started their project in 1989 to measure the drift of the Milky Way with
respect to 119 clusters of galaxies located all over the sky at distances as far as 500 million
light-years. If the motion of the Milky Way was caused by galaxies closer to us than the
distant clusters, as was then presumed to be the case, then its motion with respect to the
clusters should have been essentially identical to that with respect to the microwave
background radiation.

Because the galaxy clusters are at a variety of distances from us, galaxies in the more distant
clusters appear dimmer than the ones more nearby. However, once the various distances are
accounted for, the brightest galaxy in each cluster is always found to give off roughly the
same amount of light. Astronomers refer to such objects as "standard candles." The
distances to the clusters are estimated from how fast they are moving away from us as the
universe expands.

If the Milky Way Galaxy is drifting, however, its motion makes measurement of the
expansion speeds dependent on which direction we are looking. If the drift is not corrected
for, then the cluster galaxies will appear to vary slightly in brightness in a smooth pattern
across the sky. Postman and Lauer used images of the cluster galaxies to detect this pattern
and determine the motion of our own galaxy.

The motion of the Milky Way that Postman and Lauer measured from the distant clusters is
in a completely different direction from that inferred from the microwave background. The
most likely solution to this dilemma is that the clusters themselves are moving with an
average velocity of 425 miles per second towards the constellation of Virgo. Because of the
enormous size of the volume containing the clusters, however, this implies the existence of
even more distant and massive concentrations of matter.

Most theories explaining the structure of the universe predict that the universe should be
nearly uniform on the scale of the Lauer and Postman cluster sample. The motion of the
Milky Way and its neighbors would then be due t o concentrations of mass relatively close by.
If instead, the portions of the universe as big as a billion light-years in diameter are still
drifting with respect to the larger universe, then the universe has structure or "lumps" of
matter on much larger scales than predicted by most theories. The detection of galaxy flows
across large volumes of space should improve our understanding of how the universe came to
be organized the way we see it today.

A more provocative but probably less likely interpretation of the Postman and Lauer result is
that the large volume of clusters really is at rest, with the temperature variation of the
imicrowave background around the sky being a relic of the conditions of the Big Bang, rather
then being caused by the motion of our galaxy. In this case, the microwave temperature
variation would tell about the properties of the very early universe rather than about large
scale motions of galaxies.